In a wireless communication apparatus such as a transceiver, a power amplifier amplifies a transmission RF (radio frequency) signal so as to radiate the transmission RF signal from an antenna on the transmitting side. The power amplifier is required to maintain linearity so that the transmission RF signal falls within a spectrum mask specified by the wireless system standard. Also, the power amplifier is also demanded to reduce an odd distortion since, for example, an adjacent channel leakage power as one of the parameters specified by the wireless system standard mainly occurs due to the odd distortion of the power amplifier.
In order to improve the linearity of the power amplifier by reducing the odd distortion, for example, the Cartesian loop is used. The Cartesian loop is a feedback loop including a feedback path in which part of the power of a transmission RF signal is demodulated by a quadrature demodulator to baseband I/Q signals, and the baseband I/Q signals are fed back to inputs of a quadrature modulator of a transmitting unit.
More specifically, part of the power of a transmission RF signal output from the power amplifier is guided to the feedback path of the Cartesian loop, and is input to the quadrature demodulator via a variable attenuator and low-noise amplifier. Feedback I/Q signals obtained by the quadrature demodulator are synthesized with (added to or subtracted from) input I/Q signals. The synthesized I/Q signals are input to the quadrature modulator via a baseband amplifier. If a gain from the quadrature modulator to the quadrature demodulator is sufficiently larger than 1, a gain from the input terminal of the quadrature modulator to the output terminal of the power amplifier is decided by an attenuation amount of the feedback path of the Cartesian loop, and the linearity of the transmission RF signal depends on that of the feedback path. Therefore, high linearity of the feedback path is required.
On the other hand, since a transceiver operates using an internal battery as a power source, an IC power consumption reduction technique is indispensable so as to assure a long communication time by reducing the battery consumption. In the transceiver, the power amplifier of the transmitting unit accounts for the largest consumption power. A wireless unit used in the transceiver is highly integrated, and low- to high-frequency units can recently be manufactured by a CMOS (complementary metal-oxide semiconductor) technology suitable for cost reduction. In general, a cost reduction of the power amplifier can be attained if the CMOS structure is adopted; however, this deteriorates efficiency. The Cartesian loop is a high linearization technique for the wireless unit, as described above, and it can also be considered as a technique for achieving a highly efficient power amplifier, from another point of view. Therefore, by applying the Cartesian loop to the wireless unit of the CMOS structure, the efficiency of the power amplifier or wireless unit can be improved.
Since the Cartesian loop has a feedback circuit, how to assure operation stability is important. In order to assure the stability of the Cartesian loop, JP-A 10-136048 (KOKAI) has proposed a method of detecting the phase of feedback I/Q signals in an open state of the Cartesian loop, and controlling the phase of a local signal supplied to the quadrature demodulator in the feedback path in accordance with the phase difference between the feedback I/Q signals and input I/Q signals. However, JP-A 10-136048 (KOKAI) does not consider the variable power (transmission power) of the transmission RF signal.
When the Cartesian loop is used, it is possible to reduce the consumption power of the entire transmission apparatus even in consideration of the consumption power of the feedback circuit at the time of a high power output of the transmission RF signal. On the other hand, at the time of a low power output of the transmission RF signal, since the linearity of the power amplifier and a driver amplifier which drives the power amplifier is high, high linearization using the Cartesian loop is not required. If the Cartesian loop is active even at the time of a low power output, the efficiency drops due to the consumption power of the feedback circuit. Therefore, in order to efficiently execute transmission power control as used in a recent wireless system, it is desirable to open/close the Cartesian loop according to the transmission power in terms of a consumption power reduction.